The electrical and electronics related industry, which is regarded as the core of modern industry, is drastically developing in related markets and technology, and has very high added value, and is thus essential for modern society. Also, the related material industry, which together with the development of such electrical and electronics related industry is also being considered important, is backing up the advancing electronics industry.
The reason why thermosetting resins which are expensive are used in the field of electrical and electronics industry is that they have superior thermal, mechanical and electrical properties by curing, compared to those of thermoplastic resins. In particular, epoxy resins are mainly used as matrix resins because of their excellent electrical and mechanical properties in the field of copper clad laminate sheets (CCLSs) for printed circuit boards.
Generally, in order to fabricate a printed circuit board suitable for use in electrical and electronic products, a copper clad laminate (CCL) is manufactured by impregnating glass fiber, kraft paper or nonwoven fabric with a thermosetting resin such as phenol or epoxy resin, drying the resin to a semi-cured state (B-stage) thus making a prepreg, and coating one or both sides of the prepreg with copper foil. Also, in the case where a copper clad laminate having three or more layers which is classified as a multilayer printed circuit board is manufactured, an inner layer having wiring is formed on a double-sided copper clad laminate, and an outer layer is formed thereon using a prepreg and copper foil. Recently, as the sizes of the pads of the printed circuit board are reduced and the circuit width is reduced for fine pitch, high circuit density and high integration, it is common for micro via holes to be formed between the inner and outer copper clad laminates. The formation of such via holes may include laser drilling or plasma processing. As such, when the inorganic component such as glass fiber is included in the copper clad laminate as the reinforcement thereof, processing is not easy. Hence, there are many cases in which the insulating layer is formed of only a resin component without the reinforcement, which is called resin coated copper. When the multilayer printed circuit board is fabricated using the resin coated copper in this way, the via holes may be easily formed using laser drilling and fine pitch circuitry which is more stable may be formed.
The printed circuit board having the copper clad laminate or the prepreg and the resin coated copper should be imparted with flame retardancy so as to prevent ignition upon firing. The standard for flame retardancy typically is specified as a UL rating, and a UL-94 V-0 rating is required. Thus, the resin composition used therefor should have flame retardancy.
On the other hand, the epoxy resin exhibits the properties of thermosetting resin as a result of reacting with various hardeners. The epoxy resin is generally a polymer having two or more oxirane groups as a chemical unit of the molecule.
The case where the epoxy resin is used alone seldom occurs, and the epoxy resin may be used in the form of a thermosetting material after reaction with a hardener. The typical curing system which is the most commercially available may include amine/epoxide reaction, amide/epoxide reaction, acid anhydride/epoxide reaction, phenol novolac/epoxide reaction or the like.
Among them, the phenol novolac/epoxide system, which has outstanding heat resistance, dimensional stability, chemical resistance, and superior electrical properties, is mainly employed in electrical and electronics application such as semiconductor packaging or printed circuit board fabrication. In particular, in the case of a copper clad laminate for a printed circuit board, the use of phenol novolac hardener is gradually increasing in order to increase heat resistance.
As mentioned above, a variety of electrical and electronic products including a copper clad laminate require flame retardancy. To this end, diverse flame retardant compounds are being used. For example, a halogenated epoxy resin, in particular, a tetrabromobisphenol-A (TBBA)-containing epoxy resin is mainly utilized in a copper clad laminate. Furthermore, an additive type halogen-free flame retardant such as red phosphorus may be used. However, because the additive type flame retardant may cause the problem of the properties of a copper clad laminate deteriorating, it is seldom used. The halogenated compound emits very harmful gas (HBr, HCl, etc.) upon burning, undesirably causing problems such as harmfulness to the human body and metal corrosion. Because the regulations regarding the use of halogenated compounds have intensified in the European Union and Japan, the use thereof is becoming more and more difficult. With the goal of solving such problems, the demand for phosphorus- and nitrogen-based flame retardant compounds having low toxicity is increasing.
In particular, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (hereinafter, abbreviated to “DOPO”), which is a phosphorus-based compound, is very frequently used for a copper clad laminate. Though DOPO may be used as an additive type, it may be employed in producing a phosphorus-containing epoxy resin through a reaction with the epoxy resin. When this DOPO is used as the main material of an epoxy resin composition, flame retardant properties may be imparted. The reason why DOPO is mainly used is that it has a very high phosphorus content on the order of 14.5% and has a structure in which one direction is open, thus easily exhibiting flame retardant properties. Furthermore, DOPO is advantageous because of good reactivity with an epoxide group thus facilitating the synthesis of a phosphorus-containing epoxy resin. Thorough research into flame retardant epoxy resins including DOPO has been conducted to date. However, limitations are imposed on sufficiently manifesting flame retardancy even when DOPO is contained in the epoxy resin composition, and cured products actually have low phosphorus content below an appropriate level. Thus, even in the case where the phosphorus-containing epoxy resin is used as a main material, a flame retardant system further including an additive type flame retardant is typically adopted.